The present invention relates generally to carburetion systems for internal combustion engines and more particularly to a single control fixed fuel metering internally vented float bowl carburetor with enhanced priming capacity.
Small engine carburetors may be categorized as either of the diaphragm type where pressure differentials move a diaphragm to control fuel flow to the carburetor or of the float bowl type where a valve controlling float opens and closes to maintain a preferred fuel level in a fuel reservoir or chamber within the carburetor.
In one version of the float bowl type carburetor, fuel flows from this reservoir through a fuel metering orifice into a fuel well from which that fuel is sucked up and mixed with air due to the pressure differential caused by a Venturi region in the carburetor bore or throat. A proper fuel flow rate in this variety of carburetor is facilitated by venting the top of the float bowl to a constant pressure region. This venting may be to the atmosphere external of the carburetor or to a region of relatively constant pressure close to atmospheric pressure within the carburetor bore. The latter scheme is referred to as internal venting and has the advantage that the air supplied to the vent has already passed through the carburetor air filter and the likelihood that dirt will be introduced into the system causing difficulties such as the clogging of the fuel metering orifice is reduced.
Over a period of time, the engine intake air filter becomes dirty and clogged, so as to restrict air intake into the engine and to create a pressure drop across that air filter. With an externally vented float bowl, the effect of this restriction is to cause the engine to run on fuel rich mixture with the typical symptoms of loss of power, excessive carbon build-up in the combustion chamber and fouled spark plugs. The mixture becomes excessively rich because the pressure in the float bowl, forcing fuel through the metering orifice, remains at atmospheric pressure, so the rate at which fuel is supplied to the engine remains relatively fixed while the air intake restriction reduces the amount of air drawn into the engine, creating the unduly rich situation.
In an internally vented float bowl carburetor, the result of air intake restriction is to reduce the pressure within the float bowl and diminish the rate at which fuel is supplied to the engine with this effect being somewhat more pronounced than the decrease in combustion air being supplied to that engine so that the net result is an unduly lean mixture being supplied to the engine with the typical system of overheating of the engine. The smaller the air vent opening into the float bowl becomes, the more pronounced this leaning out effect due to air intake restriction becomes.
It is common practice to supply an initially fuel rich mixture to an internal combustion engine when attempting to start that engine. In addition to the conventional choke valve, several schemes for squirting fuel into the throat of the carburetor have been devised. An automatic arrangement for accomplishing this initial priming function is illustrated in U.S. Pat. No. 3,780,996 wherein when the engine is not running, a relatively small fuel well is filled to a certain level from the float bowl by way of the fuel metering orifice and when the engine is initially cranked, part of the fuel in this fuel well is forced into the carburetor throat and thereafter the engine runs with the fuel level in the well substantially lower than that fuel level was prior to initially cranking the engine. This system provides a fixed priming charge and works well so long as the environmental temperature range in which the engine is to be used is not excessive. For example, such an automatic priming scheme is well suited to lawnmower engine installations since the range of temperatures over which the average individual will mow a lawn is fairly limited. This patented system employed a single manual control member and a single fuel supply nozzle in conjunction with a fixed fuel metering orifice and represents a very simplistic and economical carburetion system. On the other hand, this patented system is certainly limited in the range of temperatures in which it may be employed and requires a short waiting period between attempts to start the engine in order to allow time for fuel to again fill the fuel well.
An improvement on the aforementioned U.S. Pat. No. 3,780,996 is illustrated in U.S. Pat. No. 4,203,405 wherein the advantages of the earlier patented device are retained while adding the capability of manual priming of the system. In this improvement, a flexible primer bulb may be depressed to increase the pressure on the surface of the fuel within the fuel well, forcing that fuel upwardly through a nozzle tube and into the throat of the carburetor. This later patented system may be operated in an automatic prime mode as with the earlier patented system, or preparatory to starting, the primer bulb may be depressed, forcing a first charge of fuel into the carburetor throat, and then, depending upon the time between primer actuation and starter actuation, a second at least partial fuel charge is introduced by the automatic priming aspect when the engine is cranked. Both of these patented systems require a time lag between priming attempts in order to allow time for fuel to re-enter the fuel well through the metering orifice. Thus, the priming capacity of this later patented device remains somewhat more limited than desired.